Prototype code to check for missing extensions

This is useful in general, but will also provide clear error text if
trying to fork without the posix or pcntl extensions

lib/AbstractException.php

@@ -14,6 +14,7 @@ abstract class AbstractException extends \Exception {
         "Exception.arrayEmpty"                        => 10002,
         "ExceptionType.strictFailure"                 => 10011,
         "ExceptionType.typeUnknown"                   => 10012,
+        "Exception.extMissing"                        => 10021,
         "Lang/Exception.defaultFileMissing"           => 10101,
         "Lang/Exception.fileMissing"                  => 10102,
         "Lang/Exception.fileUnreadable"               => 10103,

lib/Arsse.php

@@ -28,4 +28,18 @@ class Arsse {
         static::$db = static::$db ?? new Database;
         static::$user = static::$user ?? new User;
     }
+
+    public static function checkExtensions(string ...$ext): void {
+        $missing = [];
+        foreach ($ext as $e) {
+            if (!extension_loaded($e)) {
+                $missing[] = $e;
+            }
+        }
+        if ($missing) {
+            $total = sizeof($missing);
+            $first = $missing[0];
+            throw new Exception("extMissing", ['first' => $first, 'total' => $total]);
+        }
+    }
 }

locale/en.php

@@ -80,6 +80,11 @@ return [
      }',
      // indicates programming error
     'Exception.JKingWeb/Arsse/ExceptionType.typeUnknown'                   => 'Normalization type {0} is  not implemented',
+    'Exception.JKingWeb/Arsse/Exception.extMissing'                        => 'The "{first}" PHP extension {total, plural, offset:1
+        =0 {is}
+        =1 {and one other extension are}
+        other {and # other extensions are}
+        } not installed or not enabled.',
     'Exception.JKingWeb/Arsse/Lang/Exception.defaultFileMissing'           => 'Default language file "{0}" missing',
     'Exception.JKingWeb/Arsse/Lang/Exception.fileMissing'                  => 'Language file "{0}" is not available',
     'Exception.JKingWeb/Arsse/Lang/Exception.fileUnreadable'               => 'Insufficient permissions to read language file "{0}"',